fix(clipboard): correct blockwise register width computation (#35038)

(cherry picked from commit 95dfb063da)

src/nvim/ops.c

@@ -6615,10 +6615,17 @@ void finish_yankreg_from_object(yankreg_T *reg, bool clipboard_adjust)
     }
   }
 
+  update_yankreg_width(reg);
+}
+
+/// Updates the "y_width" of a blockwise register based on its contents.
+/// Do nothing on a non-blockwise register.
+static void update_yankreg_width(yankreg_T *reg)
+{
   if (reg->y_type == kMTBlockWise) {
     size_t maxlen = 0;
     for (size_t i = 0; i < reg->y_size; i++) {
-      size_t rowlen = reg->y_array[i].size;
+      size_t rowlen = mb_string2cells_len(reg->y_array[i].data, reg->y_array[i].size);
       maxlen = MAX(maxlen, rowlen);
     }
     assert(maxlen <= INT_MAX);
@@ -6720,15 +6727,7 @@ static bool get_clipboard(int name, yankreg_T **target, bool quiet)
     }
   }
 
-  if (reg->y_type == kMTBlockWise) {
-    size_t maxlen = 0;
-    for (size_t i = 0; i < reg->y_size; i++) {
-      size_t rowlen = reg->y_array[i].size;
-      maxlen = MAX(maxlen, rowlen);
-    }
-    assert(maxlen <= INT_MAX);
-    reg->y_width = (int)maxlen - 1;
-  }
+  update_yankreg_width(reg);
 
   *target = reg;
   return true;

test/functional/api/vim_spec.lua

@@ -1586,6 +1586,17 @@ describe('API', function()
       eq("Invalid 'type': 'bx'", pcall_err(api.nvim_put, { 'xxx', 'yyy' }, 'bx', false, true))
       eq("Invalid 'type': 'b3x'", pcall_err(api.nvim_put, { 'xxx', 'yyy' }, 'b3x', false, true))
     end)
+
+    it('computes block width correctly when not specified #35034', function()
+      api.nvim_put({ 'line 1', 'line 2', 'line 3' }, 'l', false, false)
+      -- block width should be 4
+      api.nvim_put({ 'あい', 'xxx', 'xx' }, 'b', false, false)
+      expect([[
+        あいline 1
+        xxx line 2
+        xx  line 3
+        ]])
+    end)
   end)
 
   describe('nvim_strwidth', function()

test/functional/provider/clipboard_spec.lua

@@ -537,7 +537,7 @@ describe('clipboard (with fake clipboard.vim)', function()
       eq('textstar', api.nvim_get_current_line())
     end)
 
-    it('Block paste works correctly', function()
+    it('block paste works correctly', function()
       insert([[
         aabbcc
         ddeeff
@@ -550,6 +550,20 @@ describe('clipboard (with fake clipboard.vim)', function()
         ddeeddeeff
       ]])
     end)
+
+    it('block paste computes block width correctly #35034', function()
+      insert('あいうえお')
+      feed('0<C-V>ly')
+      feed('P')
+      expect('あいあいうえお')
+      feed('A\nxxx\nxx<Esc>')
+      feed('0<C-V>kkly')
+      feed('P')
+      expect([[
+        あいあいあいうえお
+        xxx xxx
+        xx  xx]])
+    end)
   end)
 
   describe('clipboard=unnamedplus', function()